Throughout the following specification, the term vessel collectively includes, but is not limited to, a pipeline, pipeline section, column, column section, heat exchanger silo, and heat exchanger silo section; and, unless otherwise specified, aspects of the invention are applicable both to pre-installation quality assurance testing as well as to post-installation vessel fault, defect, and anomaly detection.
In order to maintain substantial fluid flow through a fluid transport vessel, internal vessel characteristics need to be monitored so that defects, obstructions, and other anomalies in the vessel can be detected and repaired efficiently, or in the case of quality assurance testing, discarded. In addition to manufacturing defects and other anomalies, such as obstructions, affecting fluid flow in the vessel, stresses imposed on the vessel in response to changes in fluid pressure can lead to structure fatigue and crack formation. Frequently, companies must endure substantial monetary costs and schedule delays due to the detection and repair of these vessel anomalies.
In order to satisfy processing requirements, minimize energy losses and increase worker safety, it is often desirable to attach an external layer of thermal insulation around the outside diameter of individual fluid transport vessels. As discussed in more detail below, the insulation layer tends to complicate the vessel inspection process, making it difficult to obtain fast and accurate information about vessel defects, such as corrosion under insulation, situated underneath the protective layer of insulation. Accordingly, prior inspection methods have not been entirely satisfactory in detecting vessel defects, while minimizing inspection costs and scheduling delays.
The detection of defects in vessels has been made by resorting to different solutions, for example ultrasonic inspection systems, where an internal invasive device crawls the length of the vessel while emitting ultrasonic probing pulses towards the vessel wall and receiving the reflected ultrasonic pulses in order to inspect the vessel wall for anomalies. This crawling device, typically referred to as a “pig”, poses a serious blockage to the normal fluid flow through a vessel and may require several days for the inspection of a lengthy vessel, decreasing vessel output capacity and production. Furthermore, the amount of data a pig can record, the life of its battery, and the wear of its components from crawling the vessel all limit the usefulness of the pig.
In a typical pulse-echo type of ultrasonic inspection system, an electronic signal generator is provided which generates pulses or periodic wave trains and a sending transducer responds thereto to emit a burst of ultrasonic energy. A couplant is required to transfer energy from the sending transducer to the test piece. A receiving transducer is provided to receive and convert energy reflected back on the interior of the test piece and develop corresponding electrical signals. In many cases, the same transducer is used as both a sending transducer and a receiving transducer. When a separate receiving transducer is provided, a couplant is required between it and the test piece. A display or indicating device, typically a cathode ray tube, is associated with the detector means to produce indications of reflections from internal flaws in the test piece.
Another conventional ultrasonic inspection approach is measuring the acoustic signature of a vessel to detect vessel anomalies. This technique sometimes involves hitting the vessel on its side with a hard object, such as a hammer, and then measuring the acoustic signature of the vessel. Anomalies often alter the acoustic signature of a vessel as compared to a vessel with no such anomalies. However, the magnitude of the anomaly that may be detected is dependent upon the wavelength of the waveform transmitted along the vessel, and sound waves generally have longer wavelengths than some other waveforms. Therefore, this technique typically fails to detect smaller-sized anomalies in a vessel and is relatively ineffective in pre-installation quality assurance testing.
It is important to note that the above mentioned ultrasonic inspection systems have not been entirely satisfactory with respect to the inspection of insulated vessels. For example, it is known that the amount of energy reflected at an interface between two media is a function of differences in the acoustic impedances in the two media. Since there is a large difference between the acoustic impedances in the insulation layer and that in most solids, especially metal vessels, a very high proportion of the sound wave energy generated by an ultrasonic inspection system is reflected at the vessel-insulation interface, resulting in a very low proportion of energy being transmitted to the ultrasonic detector for the detection of defects. To avoid this unsatisfactory result, the system operator would be required to perform the costly and labor intensive step of removing the protective layer of insulation from the outer surface of the vessel prior to commencing the ultrasonic inspection process.
To overcome this limitation, attempts have been made to attach X-ray equipment to an internal crawling device for the radiographic inspection of a vessel. In contrast to sound waves, X-rays, being electromagnetic waves, are not reflected by the insulation layer surrounding the vessel. To the contrary, X-rays propagate directly through the insulation layer, impinging on an X-ray detector, thereby generating an output signal for the detection of defects. In this way, it is unnecessary to remove the insulation layer from the vessel in order to conduct the X-ray inspection, allowing the inspection system to detect vessel anomalies or “corrosion under insulation” that occur when water or other destructive agents become trapped between the insulation layer and outer surface of the vessel.
One of the disadvantages of this type of X-ray machine is that the wheels tend to climb the tangential sidewalls of the vessel, causing the carriage to rock or overturn while it is traveling through the vessel. Such rocking motion also makes it difficult to properly align the attached radiation detector with the external radiation source as well as making it difficult to locate and position the radiation detector proximate the specific zone under inspection. The failure to maintain proper alignment between the source and detector adversely impacts the quality and accuracy of the inspection data. Moreover, as mentioned above, internal or invasive crawling devices pose a serious blockage to the normal fluid flow through a vessel, decreasing vessel output capacity and production.
Attempts have also been made to attach X-ray equipment to external crawling devices for the radiographic inspection of a pipe. For example, U.S. Pat. No. 5,698,854 entitled METHOD AND APPARATUS FOR INSPECTING PIPES, discloses an external X-ray scanning device that moves along the axial direction of the pipe while emitting X-ray radiation toward a plurality of detectors arranged on the opposite side of the pipe, for measuring the thickness of a pipe without the insulation being removed. However, the apparatus is not adapted to easily traverse past pipeline intersections, nor is the system capable of inspecting complex vessel structures such as heat exchangers, as discussed in more detail below.
Another approach to vessel inspection that has been proposed involves the use of radiographic film to capture images of the vessel. Such systems typically require large amounts of film, and are relatively slow since the film must be removed and developed before the images can be examined. Replacing the film with an X-ray detector is an alternative to X-ray film, but systems of this sort likewise require precise alignment of the X-ray source and detector with respect to each other and the vessel. As mentioned above, precise alignment has been heretofore difficult to achieve, especially given the immense size and length of vessels. Accordingly, it is desirable to produce a system that is capable of providing precise alignment between the source and detector, in a non-invasive manner, for the fast and accurate radiographic inspection of fluid transport vessels.
Another approach to the radiographic inspection of fluid transport vessels proposes the use of digital or CMOS radiation detectors for the inspection system. In view of the danger which radiation presents to the personnel handling the inspection equipment, digital or CMOS detectors are not entirely satisfactory for radiographic inspection systems. For instance, digital or CMOS detectors are relatively insensitive or “hard of hearing”, therefore requiring the radiation source to emit relatively high levels of radiation for the generation of a detectable output signal. Accordingly, it is desirable to produce a radiographic inspection system that does not require such high levels of source radiation in order to generate a detectable output signal.
Turning now to the X-ray inspection of hollow fluid transport vessels, standard two-dimensional X-ray images are generally sufficient to expose any structural defects or faults in the outer vessel wall surface. On the other hand, for more complex inspection requirements, such as for the inspection of the internal structure of heat exchanger vessels, such two dimensional images have well-known limitations. For example, with standard X-rays, the constructed image shows every surface in the X-ray path projected onto a flat plate. This makes it hard to study or inspect in great detail the independent characteristics of individual components or objects in the X-ray path. Moreover, with standard X-ray techniques, one has a limited choice of viewing angles; thus, it is not feasible to obtain an elemental cross-sectional view of the vessel under inspection.
The limitations of standard X-ray imaging have largely been overcome through the development of a Computed Tomography (CT) or CT scanning technology. A conventional X-ray CT scanner system generally comprises: an X-ray tube for radiating a flat, fan-shaped X-ray beam; and an X-ray detector arranged in opposition to the X-ray tube for the detection of the X-ray beam; and either a gantry to which the source and detector are attached to rotate about the object in question, or a part manipulator which has of a rotation table which can rotate the part, leaving the source and detector stationary. The object to be scanned is placed between the X-ray tube and the X-ray detector, and the X-ray tube and the X-ray detector are rotated in the same direction and at the same angular velocity, with the object as the center of rotation. During the rotation, X-ray projection data representing various-direction images of the object is collected on the basis of the X-rays detected by the X-ray detector. After the X-ray projection data is collected in a sufficient amount, it is analyzed by a computer to calculate the X-ray absorption coefficient at each voxel (volume element) in a plane slicing the object. In accordance with the absorption coefficient voxel data, a format suitable for rendering and/or analysis is produced such as gray scale or false color image.
In general, X-ray CT technology has achieved widespread use in the medical field for collecting X-ray projection and diagnostic data with respect to a patient being examined. The technology has met with more limited application in industry, especially with respect to the inspection of heat exchanger and other fluid transport vessels. Accordingly, it is desirable to produce a system that is capable of detecting the internal characteristics of relatively simple hollow fluid transport vessels and of relatively complex heat exchanger vessels in a non-invasive manner. It is also desirable to inspect insulated vessels in a fast, continuous, and cost effective manner, as well as to accurately detect smaller-sized anomalies in fluid transport vessels.
The foregoing has outlined a need for an improved system for the inspection of hollow fluid transport vessels as well as for the inspection of relatively more complex heat exchanger vessels. It is therefore desirable to have a fast, accurate, safe, and cost effective inspection system that is capable of detecting, in a non-invasive manner, detailed internal characteristics of fluid transport and heat exchanger vessels with minimal vessel production downtime.